Beam steering has a number of applications. Of major significance is its application to the field of telecommunications. The geographic area serviced by a wireless telecommunications system is partitioned into a number of spatially-distinct areas called "cells." Each cell usually has an irregular shape (though idealized as a hexagon) that depends on terrain topography. Typically, each cell contains a base station, which includes, among other equipment, radios and antennas that the base station uses to communicate with wireless terminals in that cell. Due to instantaneous geographic variations in communications traffic, it may be desirable, at times, to adjust the geographic coverage of a particular base station. This can be accomplished by beam steering.
The free-space distribution of the electromagnetic signal, radiated by a base station antenna, is determined by the antenna radiation pattern. This antenna radiation pattern is usually characterized by one main lobe and several side lobes in the azimuth and elevation planes. In most cases, it is desirable to have a very narrow main lobe, also called an "antenna beam", in one or both angular dimensions. The advantage is that a narrow antenna beam is very directive, and the angular power density in the main lobe is very high. The enhancement of main-lobe power density with shrinking beam width is also called "antenna gain".
If the beam width of an antenna is very small, it becomes sensitive to proper physical adjustment. This is important because it is often necessary to change the angular position of the antenna beam ("beam steering") or to modify the entire radiation pattern of an antenna over time ("beam shaping", e.g., change of beam width etc.). All this makes implementation of remote beam steering/beam shaping capabilities into an antenna panel favorable.
A high-gain antenna (i.e. narrow beam) usually consists of an array of radiating antenna elements implemented into a flat panel array. The flat panel further incorporates a feed network that distributes the radio frequency ("RF") power to the radiating elements. The number of array elements in each physical dimension translates into antenna gain in the corresponding angular dimension. The more elements and the higher their spacing, the higher the maximum gain achievable, i.e., the smaller the beam width. The final beam form and position of such an array can be adjusted by varying the relative signal amplitude and signal phase of all radiating elements. In most cases, however, it is sufficient, to only tune the signal phase in each radiating element. Such a signal-phase adjustment can be accomplished by implementing phase shifters into the signal lines to the radiating elements or into the feed network.
The appropriate phase shifter design depends on the type and application of the particular antenna. In telecommunications, the highly competitive market demands low-cost solutions of small size. The lack of cost intensive hermetic enclosures in the outdoor environment further requires high stability against varying weather conditions, temperature cycling, moisture, and corrosion. Moreover, compatibility with high power levels is required (up to 200 W average per antenna panel). This further means high linearity with respect to the RF-signal power. For passive devices, very low insertion loss is required.
In principal, since the phase of a traveling wave in a transmission line can be adjusted by several independent parameters, there are several approaches for realizing phase shifters for radio frequencies. The change in phase .phi. experienced by an electromagnetic wave of frequency .function. propagating with a velocity v through a transmission line of length L is given by the expression: EQU .phi.=2.pi..function.L/c.sub.tr,
where .function. is the signal frequency, c.sub.tr the propagation velocity in the transmission line, and where c.sub.tr is determined by: EQU c.sub.tr =c.sub.o /(.epsilon..sub.eff.mu..sub.eff).sup.1/2,
and where c.sub.o is the vacuum velocity of light, and .epsilon..sub.eff and .mu..sub.eff are the effective dielectric constant and magnetic permeability of the propagation medium, respectively. The signal phase .phi. can therefore be changed by either altering L, .epsilon..sub.eff or .mu..sub.eff. Further, variable inductors or capacitors can be implemented into the line, which allows phase adjustment due to their variable reactance.
There are various designs of phase shifters known that exploit one or more of these effects One type of phase shifter utilizes switchable delay lines with different lengths. Such phase shifters are big, heavy, and expensive. Further, only discrete steps in the phase shift are possible. A second type of phase shifter, called line-stretcher phase shifters, utilize coaxial transmission line that are extendable in a telescope-type fashion. This, however, requires sliding-contacts and is therefore very sensitive to corrosion.
A third type of phase shifter uses solid state electronics such as varactor diodes. These are not, however, compatible with high power levels due to inherent nonlinearities. Active solid state solutions require power amplifiers on the tower-top, which are big, heavy, and expensive. Solid state solutions are, for the most part, only practical for receive antennas where the power levels are very small.
Phase shifters using Ferri-magnetic materials ("Ferrites") utilize the change of .mu..sub.eff by applying a direct current magnetic field. They are large, heavy and expensive. More recently developed thin-film techniques are much lighter, but they are nonlinear at high power levels. There are also phase shifters that use the mechanical motion of dielectric material into the electrical field lines. The effective relative phase shift is very small for materials with low dielectric constants leading to large-sized phase shifters. For high-dielectric materials, a significant impedance mismatch occurs at the interface to the dielectric loaded region, which causes an undesirable return loss. Solutions with high dielectric materials are further prone to power loss into dielectric resonance modes. As such, all of the prior art solutions have drawbacks that make them unsuitable for a implementation in telecommunications.